In a modern electrical power grid, power is distributed from power lines through transformers to consumers of power, such as industrial facilities commercial facilities, businesses and residences. The resulting distribution network often requires those who service it to identify which power line is the right one and whether it is functioning properly. For example, there may be an unwanted ground somewhere on a floating delta three-phase power line that is miles long. Lineman must determine which line and where on that line the ground has occurred. In another example, a homeowner may complain about unsteady electrical power from an outlet. The source of the problem may be in the power supply between the outlet and the fuse box or between the fuse box and the nearest transformer, or farther down the line. In still other examples, cables from the fuse box to the transformer outside may be shorted or failing, or a fuse box may not be labeled correctly so that, when one apartment resident of many is moving and the electricity to that apartment is to be turned off, a utility service technician will need to identify which cables at the transformer serve the meter corresponding to that apartment.
An electric utility may want to verify which power line at a substation transformer is connected to the electrical source and which is connected to a load, or it may want to know what part of the load it is supplying on a grid supported by several other electric utilities, particularly if there is a new industrial facility soon to come on line.
In each of these and other examples, electrical power is being transferred —or should be transferred—from one part of the electrical distribution or transmission network to another. In order to make the network work, linemen and service technicians need to be able to identify whether power is being transferred, over which lines it is being transferred and perhaps how well it is being transferred and how much electrical power is being transferred.